A New Nonlinearity for Superconducting Circuits

September 5, 2024• Physics 17, s107

Researchers have isolated a high-order term in the behavior of a Josephson junction, which could lead to longer-lived superconducting qubits.

Qubits based on superconducting circuits come in various forms, but one thing they have in common is their reliance on Josephson junctions, which give a circuit the nonlinearity necessary to generate qubit states. Simon Messelot from CNRS Grenoble, France, and his colleagues have now built a superconducting circuit whose nonlinear response differs from that of conventional Josephson-junction-based circuits [1]. The circuit design could lead to superconducting qubits with much longer coherence times.

A conventional Josephson junction is a short constriction or insulating section within a superconducting circuit. Current flows across the junction by quantum tunneling, and the magnitude of this current is related to the phase difference $𝜑$ between the wave functions of the superconductor on either side of the junction. In traditional junctions, the current-phase relation (CPR) is nonlinear based on a sin( $𝜑$) dependence. But in recent years, researchers have developed junctions based on semiconductors and two-dimensional materials whose CPR can include higher-order terms such as sin( $2𝜑$). Nevertheless, the behavior is typically dominated by the lowest order sin( $𝜑$) term.

Messelot and colleagues fabricated a superconducting circuit incorporating three graphene-based Josephson junctions. These junctions were arranged to form two superconducting quantum interference devices (SQUIDs), which are circuits in which the junctions’ $𝜑$ values are sensitive to magnetic fields. By applying a magnetic field and controlling the voltage across every junction, the team suppressed the sin( $𝜑$) term in one of the SQUIDs, causing its CPR to be dominated by sin( $2𝜑$).

A superconducting circuit governed by sin( $2𝜑$) has a more complex energy landscape, with two energy minima. A qubit whose states are defined by these minima would be longer lived and easier to manipulate than a conventional superconducting qubit.

–Marric Stephens

Marric Stephens is a Corresponding Editor Physics Magazine based in Bristol, UK.

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